Solid-state imaging readers have been used in supermarkets, warehouse clubs, department stores, and other kinds of retailers to electro-optically read one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, on products to be purchased, each symbol having a row of bars and spaces spaced apart along one direction, and also for processing two-dimensional symbols, such as Code 49, on such products, as well as other items. The structure of Code 49, which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol, is described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure for increasing the amount of data that can be represented or stored on a given amount of surface area is known as PDF417 and is described in U.S. Pat. No. 5,304,786.
A typical imaging reader has an imaging assembly including a solid-state imager having a one- or two-dimensional array of cells or photosensors lying in a sensor plane, the photosensors corresponding to image elements or pixels in a field of view of the array, and imaging optics having an imaging plane at which, or in the proximity of which, an image of a symbol is optimally imaged and read. In optics, the imaging plane is a plane where the sharpest image of an object is formed by an imaging lens. Herein, the imaging plane is located in the object space, that is, where the symbol is located in a range of working distances relative to the imaging reader. The imaging plane or object (symbol) plane is conjugated to the sensor plane, which yields the sharpest image on the array. The image of the symbol at the conjugated distance projected on the array by the imaging optics is the sharpest and has the best possible quality for reading the symbol. The imaging assembly is similar to that used in a digital camera. The array may be a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, together with associated circuits for producing electronic signals corresponding to a one- or two-dimensional array of pixel information over the field of view. A microprocessor is used to analyze and decode the captured image of the symbol. The array may be used for capturing a monochrome image of a symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. The array may have multiple buried channels for capturing a full color image of a target as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
Yet, the use of an imaging reader, especially a handheld movable reader, for reading symbols located anywhere within a range of working distances relative to the reader has proven to be challenging. An operator cannot see exactly whether a symbol is within the field of view of the array during reading, or know whether the symbol is located at, or in close proximity to, the imaging plane, for optimum reading within the working range. This is especially true for reading a symbol of high density. The symbol must lay preferably entirely within the field of view, as well as at, or near, the imaging plane within the working range, to be successfully and rapidly imaged, decoded and read. It is not uncommon for the operator to repeatedly move the portable reader in multiple side-to-side, up-and-down, and back-and-forth, directions and repeatedly aim the portable reader at a single symbol several times before an indicator advises the operator that the symbol has been successfully imaged and read, thereby slowing down transaction processing and reducing productivity.
This blind aiming at the symbol is easier if the symbol is relatively small or is far away from the reader, because then the chances that the symbol will lay within the field of view are greater. However, in most cases, this blind aiming at the symbol is difficult to overcome, especially when the position and orientation of the symbol are variable, which is the case for most applications.
To help overcome this blind aiming problem, an aiming light arrangement is typically mounted in the reader, for projecting a visible aiming light pattern to visually target the symbol within the field of view and, thus, advise the operator which way the reader is to be moved in order to position the aiming light pattern on the symbol, typically at the center thereof, prior to reading. As advantageous as such aiming light arrangements are, they have proven to be less than satisfactory in certain situations. For example, the aiming light pattern does not typically provide information about how far from, or close to, the reader the symbol should be located for best reading performance. The aiming light pattern typically indicates the overall size of the field of view, but does not guide the operator to the imaging plane at which the symbol is best read. The aiming light arrangement is designed so that the aiming light pattern remains sharp and focused throughout the working range and, as a result, the operator is left with no guidance as to where the optimum reading distance is within the range.